The invention relates to a process for calcining mineral raw materials, such as limestone, dolomite, or magnesite, in a uniflow regenerative shaft furnace having at least two shafts which are connected to each other through a transfer or annular duct with each shaft being alternately operated as the calcining or uniflow shaft and as the counterflow shaft, wherein the calcined raw material is cooled in cooling zones of both shafts.
For calcining mineral raw materials, a regenerative process described in U.S. Pat. No. 3,074,706 is known which is frequently used for the construction of uniflow/counterflow shaft furnaces having at least two shafts. This type of furnace is simple in its operation and, aside from a low energy consumption, it has the significant advantage of providing a good soft burning of the calcined raw material, while also being capable of producing medium and hard burning.
A large portion of the material produced in such uniflow/counterflow shaft furnaces is utilized in steel mills. For metallic and quality reasons, new and restricting requirements are imposed upon these steel mills. However, these can only be met when the materials used in the plants meet certain requirements. Among these is the requirement that the lime produced in the calcining furnaces have a very high degree of desulphurization, for example, about 90%.
Experiments have shown (Transactions AIME, volume 254, March 1973, pages 28-33) that it is possible to achieve a desulphurization of sulphur-containing limestone or to prevent an absorption of sulphur introduced with the fuel in the calcined lime, if a reducing atmosphere can be adjusted in the calcining zone of the furnace or in the calcining unit. For a given composition of the flue gases, in accordance with the thermal dynamic basic equations, the degree of desulphurization increases with increasing temperature in dependence upon the maximum partial pressure of the SO.sub.2 generated in the flue gases during the desulphurization.
If this finding is to be utilized in the aforementioned uniflow/counterflow lime shaft furnace, it would be possible to adjust a reducing atmosphere in the calcining shaft of such a furnace by means of combustion with less than stoichiometric amounts. However, due to the fact that the cooling air is mixed with the flue gases coming from the calcining shaft and with CO.sub.2 expelled from the limestone in the calcining shaft, an oxygen-containing atmosphere is obtained in the counterflow shaft. As a result of the equilibrium, this oxygen-containing atmosphere may again lead to a partial sulphurization of the calcined lime. Accordingly, it is not always possible to achieve the desired high degree of desulphurization.
The present invention proceeds from this state of the art and is directed toward development of a process of the type described above in such a manner that the flue gases generated in the uniflow or calcining shaft can be conveyed to the counterflow shaft without oxygen.